1. Field of the Invention
The present invention relates to a multi-area focus detecting device in which the focus condition of the objective lens is detected not only with respect to an axial focus detection area located on the optical axis of the objective lens, but also with respect to at least one off-axial focus detection area located far from the optical axis thereof. 2. Description of the Prior Art
A conventional multi-area focus detecting device is shown in FIG. 12. In this device, a pair of light fluxes used for detecting the focus condition with respect to the axial focus detection area, containing a pair of principal rays l1 and l2, are focused on a CCD (charge-coupled device) line sensor P0, through an aperture opening E0 provided on a field mask FM, a condensor lens L0, a pair of aperture openings A1 and A2 provided on an aperture mask AM, and a pair of re-imaging lenses L1 and L2 formed on a re-imaging lens plate L, and a pair of secondary images are formed on the CCD line sensor P0. On the other hand, two pairs of light fluxes used for detecting the focus condition with respect to two off-axial focus detection areas, containing two pairs of principal rays l3 and l4, l5 and l6 respectively, are focused on CCD line sensors P01 and P02 respectively, through a pair of aperture openings E01 and E02, condenser lenses L01 and L02, two pairs of aperture openings A3 and A4, A5 and A6, and two pairs of re-imaging lenses L3 and L4, L5 and L6, respectively, and respective pairs of secondary images are formed on CCD line sensors P01 and P02, respectively. The focus condition of the objective lens TL can be detected by means of detecting three pairs of the secondary images.
An AF (auto-focus) sensor module AF is composed of the aforementioned field mask FM, condenser lenses L0, L01, and L02, aperture mask AM, re-imaging lens plate L and CCD line sensors P0, P01, and P02.
However, respective light fluxes used for detecting the focus condition become close to each other in order to make the AF sensor module AF more compact. Therefore, any light rays, which includes light rays to be incident on the other CCD line sensors, may be incident on the CCD line sensors, resulting in the deterioration of the precise focus detection capability.